The present invention relates generally to linear motors, and more particularly, to a linear vibration motor that is suitable for use in an electric shaver, and a method for controlling vibration of the linear vibration motor. This application claims priority under 35 U.S.C. .sctn. 119 of Japanese Patent Application No. 09-041238, filed Feb. 25, 1997, the disclosure of which is incorporated herein by reference.
Japanese Unexamined Patent Publication No. 2-52692 discloses a linear motor that is utilized as a source of reciprocating vibration for a reciprocating type electric shaver. This linear vibration motor is a single-phase synchronous motor that includes a reciprocator (or needle) comprised of a rod-like permanent magnet, and a stator having U-shaped iron cores around which coils are wound. A dc voltage having a frequency two times greater than ac frequency is supplied to the coils by a full-wave rectifying circuit to thereby induce reciprocating movement (oscillation) of the reciprocator, and thereby generate vibration.
The electromagnetic force that is required to induce the reciprocating movement of the reciprocator (or needle) is greater than desired. If the reciprocator were supported with a spring to thereby form a spring vibration system (including the reciprocator), and the spring vibration system driven at a frequency equal to a characteristic (resonant) frequency thereof, it would be possible to reduce the amount of energy necessary to drive the spring vibration system. However, the amplitude of the reciprocating vibration could not be kept stable when the spring vibration system is loaded.
As a solution to this problem, there has been proposed a linear motor including a stator comprised of an electromagnet or permanent magnet, a reciprocator comprised of a permanent magnet or electromagnet and supported with a spring, a detection system that detects the displacement, velocity, or acceleration of the reciprocator and produces a feedback signal indicative thereof, and a control system that controls the amount of electric power supplied to a coil of the electromagnet in response to the feedback signal. With this linear motor, even when a characteristic (resonant) frequency of the vibration system varies, e.g., due to changes in load, the control system automatically varies the amount of electric power supplied to the coil of the electromagnet in such a manner as to drive the vibration system at the current resonant frequency (i.e., to drive the vibration system in resonant condition). This is possible because the variation of the resonant frequency is related in a known manner to the displacement, velocity and acceleration of the reciprocator, at least one of which is detected by the detection system.
However, when the vibration system is heavily loaded, with the result that the amplitude of the reciprocating vibration is considerably reduced, the detection system is rendered inoperative, thereby making it impossible to drive the vibration system in resonant condition. In addition, it takes a considerable amount of time for the vibration system to return to resonant condition even after it is unloaded, thereby resulting in a considerable reduction in the efficiency of the linear motor.
Based on the above, it can be appreciated that there presently exists a need in the art for a linear vibration motor which overcomes the above-described drawbacks and shortcomings of the presently available technology. More particularly, there is presently a need for a linear vibration motor which has the capability of appropriately driving the vibration system when the detection system becomes incapable of carrying out its detection function, and the further capability of rapidly returning the vibration system to resonant condition when the detection system becomes capable of carrying out its detection function. The present invention fulfills this need in the art.